1. Field of the Invention
The present invention relates to the field of liquid crystal displaying techniques, and in particular to an LED backlight source and liquid crystal display device.
2. The Related Arts
As the technology continuously progresses, the development of backlight technology of liquid crystal display device also continuously grows. The conventional backlight technology of liquid crystal display device uses cold cathode fluorescent lamp (CCFL). But as CCFL backlight has the disadvantages of poor color recovery, low light-emitting efficiency, high discharge voltage, poor discharge characteristics at low temperature, long time to reach stable grayscale, and so on, the backlight technology employing LED has been developed.
In liquid crystal display device, the LED backlight source and the liquid crystal display panel are disposed oppositely face-to-face so that the LED backlight source provides light source to the liquid crystal display panel, wherein the LED backlight source comprises a plurality of LED strings, with each LED string comprising a plurality of LEDs strung in series.
When the LED backlight source needs more LED strings to provide the backlight of high luminance, a plurality of constant-current drivers are connected in parallel to provide sufficient number of connection channels for more LED strings.
FIG. 1 is a schematic view showing a direct LED backlight source of a known technology used in liquid crystal display devices. As shown in FIG. 1, the backlight driver includes a boost converter 110, a plurality of LED strings 120 and a plurality of constant-current drivers 130.
The first constant-current driver 131 controls the connection or disconnection of the boost converter 110 to make the boost converter 110 boost the input DC voltage Vin and output boosted DC voltage to the plurality of LED strings 120. In addition, the input DC voltage Vin, after divided by the resistors R1, R2 and R3, is connected each constant-current driver. When the input DC voltage is too low (for example, a large-power appliance is connected to the power supply loop or a low input DC voltage caused by high power consumption during peak time) so that the voltage over the resistor R3 is not greater than an under-voltage protection trigger voltage, the constant-current driver will cut off the power supply, leading to shutting down the connected LED strings.
In actual application, the constant-current driver is often integrated onto a chip. Because of the manufacturing process, the under-voltage protection trigger voltage of each integrated circuit chip is not completely identical and a small variation exists. Therefore, when the input DC voltage is too low, the voltage over the resistor R3 is just about triggering the under-current protection function of a constant-current driver, and the voltage over the resistor R3 at this point is not necessarily larger than the under-voltage protection trigger voltages of other constant-current drivers, as a result, some LED strings of the plurality of LED strings 120 connected in parallel will shut down (i.e., OFF) while others still function normally (i.e., ON). As such, the optical quality of the LED backlight source suffers a negative impact. Also, when the input DC voltage Vin changes from low to high, the plurality of LED strings 120 connected in parallel will display the aforementioned incoherent behavior.